Method for manufacturing multilayer circuit substrate

ABSTRACT

A method for manufacturing a multilayer circuit substrate includes the step of providing an insulating substrate formed of an inorganic oxide and supporting a laminate formed by alternately laminating wiring layers of copper and insulating layers of an inorganic oxide. The uppermost layer of the laminate is constituted by a copper wiring layer. A layer of electrically conductive material capable of being subjected to wire bonding is formed by a low temperature deposition on the surface of the laminate. Subsequently, the conductive material layer is selectively removed by photoetching to allow the portion connected to part of the uppermost copper wiring layer to remain. In this manner, the pattern layer capable of wire bonding can be formed.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to a method for manufacturing a multilayercircuit substrate in which wiring layers and insulating layers arealternately laminated on an insulating substrate.

II. Description of the Prior Art

In a conventional multilayer circuit substrate in which wiring layersand insulating layers have been alternately laminated on an insulatingsubstrate formed of ceramics, gold has been used for the wiring layers.Since the gold is expensive, however, it is preferred to employ aswiring layers a less expensive metallic material such as, for example,copper. Usually, a wire bonding process has been employed to bond a leadwire to the uppermost wiring layer in the multilayer circuit substrate.However, the disadvantage to using copper is that the wire bondingprocess can hardly be applied to copper wiring layers in the multilayercircuit substrate.

The insulating layers of the conventional multilayer circuit substratehave been formed of an organic material, e.g., polyimide. When asemiconductor element such as a large-power LSI was mounted on themultilayer circuit substrate, since such an organic material hadimproper thermal conductivity, the conventional multilayer circuitsubstrate which employed the organic material insulating layer had agreat drawback in that the heat generated in the semiconductor elementcould not be rapidly dissipated at its operating time.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method which caninexpensively produce a multilayer circuit substrate which has muchbetter heat dissipation characteristics than the conventional multilayercircuit substrate.

According to a method for manufacturing a multilayer circuit substrateof the present invention, an insulating substrate which is formed of aninorganic oxide and which supports thereon a laminate wherein wiringlayers formed of copper and insulating layers formed of an inorganicoxide are alternately laminated is employed. The uppermost layer of thelaminate is the copper wiring layer. A layer of an electricallyconductive material capable of being subjected to a wire bonding processis formed by a low temperature deposition on the surface of thelaminate. Subsequently, the conductive material layer is selectivelyremoved by a photoetching method so that the portion connected to partof the copper wiring layer forming the uppermost layer of the laminateremains. In this manner, a pattern layer which is capable of beingsubjected to wire bonding can be formed.

In the multilayer circuit substrate according to the present invention,substantially all the wiring layers can be formed of copper and theinsulating layers between the respective wiring layers are formed of aninorganic oxide. Therefore, the entire circuit substrate can beinexpensively manufactured, and good heat dissipation can be provided atthe substrate. In addition, since a conductive material layer which iscapable of being subjected to a wire bonding process is formed on theuppermost copper wiring layer, the wire bonding process can be appliedto the circuit substrate without fail. In the present invention, it isimportant to form the conductive material layer by a low temperaturedeposition (which will be defined later) and to pattern the conductivematerial layer by photoetching. The low temperature deposition andphotoetching do not produce a brittle alloy between the uppermost copperlayer and the conductive material. Further, the photoetching enables amicrominiature patterning.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is best understood by reference to the accompanyingdrawings, in which:

FIG. 1 is a sectional view for explaining one embodiment of a method formanufacturing a multilayer circuit substrate according to the presentinvention; and

FIGS. 2A to 2E, 3A and 3B and 4A to 4C are sectional views showingdifferent embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in more detail withreference to the accompanying drawings. The same portions in therespective drawings are designated by the same or equivalent referencenumerals in FIGS. 1 through 4C.

FIG. 1 is a sectional view illustrating one embodiment of a method formanufacturing a multilayer circuit substrate according to the presentinvention. In this embodiment, copper wiring layers and insulatinglayers are formed by utilizing a printing of a paste.

As shown in FIG. 1, an insulating substrate 11 which is formed of aninorganic oxide material is first prepared. The inorganic oxide materialincludes ceramic-based materials (e.g., alumina, and mixtures of thealumina with any of 0.5 to 8 wt. % of other oxides such as borontrioxide, silicon dioxide and lead oxide), and oxide glass materials(e.g., silicon dioxide).

A copper paste and a paste of an inorganic oxide material (e.g., amixture of alumina and 8 to 80 wt. % of the total sum of silicondioxide, boron trioxide and lead oxide) are alternately printed on aninsulating substrate 11, dried and baked to form copper wiring layers12, 14 and 16 as well as insulating layers 13 and 15. The baking iscarried out in a non-oxidizing and non-reducing atmosphere, i.e., in aneutral atmosphere (e.g., nitrogen gas or argon gas). If the baking iscarried out in an oxidizing atmosphere, the copper is oxidized, with theresult that the wiring layer formed of the copper does not function as awiring layer. If the baking is carried out in a reducing atmosphere, theinorganic oxide forming the insulating layers is reduced, with theresult that the insulating layer formed of the inorganic oxide does notfunction as an insulating layer. As shown, the wiring layers 12, 14 and16 are electrically connected to each other through openings formed atthe insulating layers 13 and 15.

Subsequently, a layer of an electrically conductive material which iscapable of being subjected to a wire bonding process (i.e., a materialwhich forms a layer to which a lead wire can be bonded by a conventionalwire bonding process without fail) such as, for example, gold, silver oraluminum is formed by a low temperature deposition on the surface of thelaminate thus obtained. In this specification, "low temperaturedeposition" means a technique which forms the electrically conductivematerial layer at a low temperature that does not diffuse the copperforming the wiring layer 16 of the uppermost layer of the laminate inthe conductive material or does not alloy the copper with the conductivematerial, and includes a vapor deposition, a sputtering, an ion plating,a plating (including both an electroless plating and an electrolyticplating) known per se. In case of employing the plating, it is preferredaccording to the present invention to form by electroless plating anickel layer on the surface of the laminate prior to the formation ofthe conductive material layer, and to form an electrically conductivematerial layer on the nickel layer.

Thereafter, the conductive material layer is photoetched, i.e., aphotoresist is formed on the entire surface of the conductive materiallayer, dried, exposed to light and developed; with the remaining resistused as a mask, the conductive material layer is selectively etched, anda pattern layer 17 connected to the copper wiring layer 16 remains, asshown. This pattern layer 17 becomes a bonding pad of the multilayercircuit substrate thus obtained.

In the embodiment of the method described above, the wiring layers 12,14, 16 are formed of copper, and are therefore more economical. Sincethe uppermost wiring layer 16 is covered with the layer 17 capable ofbeing subjected to wire bonding process, no problem occurs in the wirebondability of the final multilayer circuit substrate.

Particularly, the deposition or sputtering for forming the conductivematerial layer is normally carried out at a low temperature (200° C. orless), the plating is performed at a lower temperature, and the etchingstep is also carried out at a lower temperature. Therefore, no brittlealloy is formed due to the diffusion between the gold used as theconductive material layer and the copper wiring layer, and the materiallayer 17 has sufficient strength. The printing and baking of a goldpaste can be, for example, considered in the formation of such aconductive material layer, but diffusion between the gold and the copperoccurs at a high temperature required to bake the gold paste, resultingin the formation of brittle alloy, which is not preferred as a bondingpad.

In the method of the above embodiment, the insulating layers are formedof an inorganic oxide. Therefore, its heat dissipation is for superiorto the organic material, and the insulating layers of the inorganicoxide are well-suited for mounting a large-power semiconductor element.

Since the wiring layers 12, 14 and 16 are formed by printing and bakingof the copper paste in the embodiment described above, the thickness ofthe wiring layers can be increased, a thickening step involving platingis not necessary, and no problem occurs even in the passage of a largecurrent therethrough. In particular, the thick wiring layer formed ofthe copper paste has features and advantages not only of electricalconductivity, but of preferable solderability and a resistance to solderreaching.

FIGS. 2A to 2E show sectional views for describing another embodiment ofthe method according to the present invention. In this embodiment, acopper wiring layer at least formed directly on the insulating substrateis formed of a thin copper film.

As shown in FIG. 2A, a thin copper film 22 is formed, for example, byvapor deposition, sputtering or plating, in a thickness of about 1 to 20μm on the overall surface of an insulating substrate 21 similar to theinsulating substrate 11. As shown in FIG. 2B, a photoresist is formed onthe overall surface of the thin copper layer 22, exposed and developedin a predetermined pattern so as to remove the unnecessary portion isremoved therefrom. Using the remaining photoresist 23 as a mask, thethin film 22 is selectively etched and removed. Subsequently, thephotoresist 23 is removed, for example, by an oxygen plasma, by bakingat a temperature at which the thin film 22 is not oxidized (400° to 500°C., or less), or by a resist stripper.

Subsequently, the substrate 22 is heated in the above-described neutralatmosphere at a temperature at which the thin copper film 22 reacts withthe inorganic oxide material forming the insulating substrate 22. In thecase where the substrate 21 is formed of a material which contains as amain ingredient aluminum oxide (Al₂ O₃, a copper aluminate 24 is formed,as shown in FIG. 2C, at a temperature in the range from 900° C. to1,200° C., and the thin copper film 22 is rigidly bonded to thesubstrate 21. In the case where the substrate 21 is formed of a materialwhich contains as a main ingredient silicon dioxide (SiO₂), the thincopper film 22 is similarly rigidly bonded to the substrate 21 at atemperature in the range from 750° C. to 950° C.

In this manner, a wiring pattern layer 25 which is formed of the thincopper layer 22 is formed, as shown in FIG. 2C, on the substrate 21.

Then, an inorganic oxide material paste is printed in the same manner asdescribed with respect to FIG. 1, then baked in a neutral atmosphere,and an insulating layer 26 which selectively exposes the wiring layer 25is formed (FIG. 2D). Thereafter, a second wiring layer 28 is formedthrough the printing of a copper paste and baking in a neutralatmosphere in the same manner as described with respect to FIG. 1.Subsequently, a copper paste and an inorganic oxide material paste arealternately printed and baked as described with respect to FIG. 1, asrequired to manufacture a substrate for supporting a predeterminedlaminate. Or, the second wiring layer 28 and the subsequent wiring layeror layers may be formed of thin copper films and may react with theprimary insulating layer in the same method as described with referenceto FIGS. 2A to 2C.

A bonding pad is formed by the same method as described with referenceto FIG. 1 on the uppermost copper wiring layer of the laminate on thesubstrate 21 thus obtained.

Particularly, in this second embodiment, when the thin copper film isemployed as the wiring layer, it is rigidly bonded to the underlyinginsulating layer or insulating substrate.

FIGS. 3A and 3B are sectional views for describing still anotherembodiment of the method according to the present invention. In thisthird embodiment, a copper wiring layer formed directly on theinsulating substrate is formed of a thin copper film.

As shown in FIG. 3A, an adhesive layer 32 and then a film 33 of materialcapable of forming a stable alloy with copper are formed on aninsulating substrate 31, and a thin copper film 34 is covered thereonand formed, for example, by a vapor deposition, sputtering or plating ina thickness of about 1 to 20 μm. The adhesive layer 32 is formed of ametal which has a free energy of formation of oxide larger than that ofcopper, such as titanium, vanadium, chromium, tungsten or alloys thereofin a thickness of about 200 to 3,000 Å. On the other hand, the film 33is not always necessary, but is preferred to form a stable alloy withthe copper at the subsequent baking time to rigidly bond the thin copperfilm 34 to the adhesive layer 32. The film 33 is formed, for example, ofnickel or palladium.

Subsequently, the films 34, 33 and 32 are selectively removed byphotoetching in the same manner as described with reference to FIG. 2A.Thus, a wiring pattern 35 is obtained as shown in FIG. 3B.

Then, an inorganic material paste and a copper paste are alternatelyprinted and baked in the same manner as described with respect to thesecond embodiment to produce a predetermined laminate, and a bonding padlayer is formed in the same manner as described with reference to FIG. 1on the uppermost copper wiring layer of the laminate. In the thirdembodiment described above, all the wiring layers may be formed in atwo-layer or three-layer structure of a thin copper layer and anadhesive layer, and, as required, together with a layer forming a stablealloy with copper by the method as described with reference to FIG. 3A.

In this embodiment described above, the adhesive layer is formed of amaterial which has a free energy of formation of oxide higher than thatof copper. Therefore, the adhesive layer and therefore the copper layercan be bonded more rigidly to the substrate.

FIGS. 4A to 4C are sectional views for describing still anotherembodiment of the method according to the present invention. A wiringlayer which contacts an insulating substrate is formed of a thin copperfilm.

As shown in FIG. 4A, the surface of an insulating substrate 41 iscleaned, a first thin copper film 42 is formed, for example, by a vapordeposition, ion plating or sputtering on the suface of the substrate 41in a thickness of about 100 to 4,000 Å. Subsequently, the first thincopper film 42 is oxidized in oxygen plasma. This can be performed byexciting the oxygen with microwaves and introducing it via a waveguideinto a vacuum deposition chamber.

Thus, the first thin copper film 42 is oxidized, and a second thincopper film 43 having a thickness larger than the first thin copper film42, e.g., 1 to 20 μm, is formed by a vapor deposition, ion plating orsputtering on the copper oxide film 42' as shown in FIG. 4B.

Subsequently, as shown in FIG. 4C, the thin copper film 43 and thecopper oxide film 42' are selectively removed by photoetching and bakedat a temperature at which the copper oxide 42' reacts with theunderlying substrate 41. In this case, the copper oxide 42', which isproduced by the oxidation of the first thin copper film 42, has asurface free energy higher than the ordinary bulk state, and accordinglyreacts with the substrate 41 at a relatively lower temperature, e.g.,960° C. A spinel type crystal is formed by such baking with the reactionof CuO-Al₂ O₃ or CuO-SiO₂, and the second thin copper film 43 is rigidlybonded to the substrate 41. In this case, the surface of the second thincopper film 43 is not oxidized, and accordingly exhibits a preferableadhesiveness by soldering.

In this manner, a wiring pattern 44 which is formed of a thin copperfilm is formed, as shown in FIG. 4C, on the substrate 41.

It should be noted that the oxidation of the first thin copper film 42may be accomplished by baking in an oxidative atmosphere or by treatmentwith a chemical such as hydrogen peroxide solution, instead of theoxygen plasma. The formation of the first thin copper film 42 may beperformed by a chemical plating, and the formation of the second thincopper film 43 may be performed by an electric plating.

Thereafter, a laminate is obtained by an inorganic oxide material pasteand a copper paste in the same manner as described with respect to theprevious embodiments, and a bonding pad layer is formed on the uppermostcopper wiring layer. The respective copper wiring layers may, of course,be formed in accordance with the method as described with reference toFIGS. 4A to 4C.

What is claimed is:
 1. A method for manufacturing a multilayer circuitsubstrate comprising the steps of:providing an insulating substrateformed of an inorganic oxide; successively and alternately formingwiring pattern layers and insulating layers on said insulatingsubstrate, each of said wiring pattern layers consisting of a copperlayer and an adhesive layer, said adhesive layer being deposited on saidinsulating substrate and each of said insulating layers and each of saidcopper layers being deposited on each of said adhesive layers andwherein said adhesive layer comprises a first metallic material having afree energy of a formation of oxide higher than that of copper; andforming, by a low temperature deposition, a layer of an electricallyconductive material capable of being subjected to wire bonding of saidcopper layer.
 2. The method according to claim 1, wherein said adhesivelayer is formed of titanium, vanadium, chromium, tungsten or an alloythereof.
 3. The method according to claim 1, wherein said metallicmaterial forming the stable alloy with copper is nickel or palladium. 4.The method according to claim 1, further comprising the step of forminga layer of a second metallic material on said adhesive layer for forminga stable alloy with copper.